1.Field of the Invention
The present invention relates to an image reading device, and more particularly, to an image reading device having, for example, a linear image sensor for reading image information by relative movement of an information bearing member, such as an original document, in contact with said linear image sensor, and adapted for use in a facsimile apparatus, a copying machine, an image reader or the like.
2. Related Background Art
In the field of image reading device utilizing a linear image sensor, there is already known a device for reading an original image by focusing said image onto a linear image sensor of a length of several centimeters in the main scanning direction, through a reduction optical system. However such a device requires a large optical path for reduction imaging, and cannot be easily miniaturized since the optical system requires a large space.
On the other hand, the use of an equal-size optical system employing a linear image sensor of a length substantially equal to the width of the original image allows a significant reduction in the space required for housing the optical system, thereby miniaturizing the reading apparatus.
FIGS. 1A and 1B are respectively a schematic plan view and a schematic cross-sectional view along a ling A--A in FIG. 1A, showing an example of photosensor employed in conventional image reading devices. On an electroinsulating substrate 1 composed of glass or ceramics, there are formed an auxiliary electrode 2 and an insulating layer 3, on which is formed is a photoconductive semiconductor layer 4 composed, for example, of hydrogenated amorphous silicon (hereinafter written as a-Si:H), or CdS.Se. There are further formed a semiconductor layer 5 doped with a semiconductive impurity for achieving the ohmic contact, and a pair of main electrodes 6, 7 thereon, defining therebetween a light receiving window 8 for introducing light into the semiconductor layer 4.
In the above-explained structure, when a positive driving voltage is supplied to the main electrode 7 light reflected from the original document and introduced into the semiconductor layer 4 induces a photocurrent, the of which change is read as image information. The auxiliary electrode 2 is given a suitable voltage to stabilize the output of the photosensor and to provide an output proportional to the light intensity.
The above-explained photosensor corresponds to one bit (one cell) in image reading, but a plurality of such photosensors may be linearly arranged to obtain a linear image sensor of equal image size.
However, in such a conventional image reading device, the main electrodes 6, 7 and the auxiliary electrode 2 constituting the photosensor provide a large spatially overlapping area (see FIG. IA) to form a large capacitance therebetween, thus lowering the response speed to light. Also a defect, such as pinhole, in the insulating layer 3 frequently results in a short circuit, thus deteriorating the production yield of the device.